Ocean Remote Sensing from Satellites

Ocean Remote Sensing from SatellitesTed Strub, Burt 406tstrub@coas.oregonstate.eduhttp://cioss.coas.oregonstate.edu/CIOSS/• General Background• Examples, Measuring– Surface Temperature– Ocean Color/Optics– Sea Surface Height& Currents– Surface Wind Stress• New technology

http://response.restoration.noaa.gov/Satellite images are being used to track the Gulf Coast oil spill.What can we really “see” in the ocean, using satellites?

Several NOAA, Navy andUniversity (research) models arebeing initialized with satelliteimage “interpretations”,augmented by aircraftoverflightshttp://response.restoration.noaa.gov/For the future, the U.S. isdeveloping an “IntegratedOcean Observing System” thatwill combine satellite data, inwaterdata and computer modelsof the circulation and waterproperties (similar to thecomputer weather forecastmodels).

Remote Sensing PioneersEarly Days 1888

Why Satellites?• Oceans are vast, sparsely occupied – satellites provide uniformsampling, daily-to-monthly fields.• Harsh environment for acquiring measurements• Ship time is expensive• Satellites can acquire measurements quickly over large areasBUT…• Cost – Satellites are also expensive, but “global” (international)• Risk – If satellites fail, service calls are even more expensive• Must measure through intervening atmosphere• Can’t acquire sub-surface measurements• Can only measure a half-dozen variables, at the “surface” –• But: We can measure wind forcing and response

Ships take time to “map out” an area.The blue ship tracks take ~ 1-2 weeks in the case below. Patterns of cold water(whiter shades) and currents change in 4 days between images.

The temperature field constructed from ship observations (circles,below right) over a 1-week period shows some of the features evidentin the instantaneous satellite image (below left), without the detailedfilaments and swirls caused by jets and eddies in the currents, whichthe satellite field captures (August, 2000).

Components of a passive “SST” remote sensing systemUsing IR or Microwave WavelengthsTwo types ofremote sensing:sensorPassivesignalraw datacalibration/validationprocessing /disseminationIR or μ-waveEmission

Components of an active radar “altimeter” systemFor sea level: Transmit at nadir (directly beneath satellite)Two types ofremote sensing:Activesource and sensorsignalraw datacalibration/validationprocessing /disseminationμ-wave radarReflection

Orbits Determine SamplingGeostationary Orbit - GEO36,000km altitude (wide view)Stays over same locationCan document evolving systemsHigh temporal resolutionLower spatial resolution?Not necessarily – can “stare”for longer exposures.No polar coverage“Polar” Orbit (Low Earth Orbit LEO)850-1000 km altitudeTravels nearly over polesSees almost whole globeLower temporal resolutionHigher spatial resolution?Not necessarily – limited time overeach point.

Examples of several types of orbits.The Earth’s center of mass must be in the orbital plane – so Geostationary orbitsmust be in the Equatorial plane. Low Earth Orbits are inclined from theequatorial plane. Specialized orbits are used for different purposes.• Sun-synchronous orbits cross the equator at the same time each solar day.• Low inclination orbits are used to look more closely at tropical processes.• Exact-repeat orbits fill in a grid of orbit tracks over X days, then repeat exactly.

Low Earth Orbits (“Polar Orbiting”): The orbital plane of thesatellite remains fairly constant while the earth rotates toward theeast. Subsurface tracks migrate to the west.

Orbit Animations?

What SeaWiFS (visible) sees in one day:Swaths leave gaps at low latitudes:Clouds cover >50% of the ocean and obscurevisible and IR radiation.

Specific ExamplesSea Surface Temperature (SST)Perhaps the most ‘standard’ measurement from satellitesPassive meaurement. . Traditionally used infrared (IR) emission- strong signal, obscured by cloudsMore recently using Microwave- can see through clouds, but the signal is weak- microwave also provides other data such as wind speed,water vapor, rain, iceTemperature is important because of its relationship to the heatbudget (global warming) and because it’s s diagnostic ofcurrents, upwelling etc…

Components of a passive “SST” remote sensing systemUsing IR or Microwave Wavelengthssensorsignalraw datacalibration/validationprocessing /disseminationIR or μ-waveEmission

1.0 μ 10.0 μ 1 mm 1 cm 10 cmEnergyWavelength1 cm10 GHz1 GHzPlanck’s Function in log-log plot for Spectral Radiance as a function of WavelengthAll objects emit E-M radiation (you are emitting radio waves right now) –how much and the peak wavelength depends on the temperature.

Why Use Microwave?Clouds!Two-days of InfraredTwo-days of Microwave

Surface Temperature: 1 kmIR SST: 26 Sept. 1998Bathymetry

Ocean color (chlorophyll)Passive measurementMeasures light scattered and transmitted upward through theocean surface - its ‘color’ (careful to distinguish between‘transmission’ and ‘reflection’)PROBLEM: Most of the signal (>90%) at the satellite is NOTocean color – It is atmospheric interference: sunlight that hasbeen scattered or reflected by molecules or aerosols in theatmosphere back into the satellite’s s field-ofof-view.

Components of a passive “ocean color” remote sensing systemUsing Visible wavelengthssourcesensorsignalraw datacalibration/validationAbsorption and scatteringof visible light – dependson pigments in the waterThe “color” is thewavelength that isnot absorbed!processing /dissemination

Sea-viewing Wide Field-of-view Sensor: SeaWiFS8-channels, 1km pixelMain signals: Atmosphere,reflection and ocean colorL wAbsorptionWavelengthR = log 10R s(490nm)Rs(555nm)E tChl =10 (0.2733−2.3534 R +1.1416R 2 −0.2972R 3 ) − 0.0788

Surface Temperature and Chlorophyll: 1 kmSST: 26 Sept. 1998BathymetryChl-a Pigment:26-27 Sept. 1998

Sea Surface Height (SSH)Active measurement using microwave radarPulse sent from satellite to earth, measure return timeWith appropriate processing and averaging, it is possibleto calculate:• Ocean currents, eddies (scales > 60-100 km)• Deviations in ocean surface due to bathymetry• Gradual sea level rise due to global warming• Deviations in ocean surface due to internal physicalvariability (heat, salinity)

Components of an active radar “altimeter” systemFor sea level: Transmit at nadir (directly beneath satellite)source and sensorsignalraw dataprocessing /disseminationcalibration/validation

Active Radar – Altimeter: Measures SSH ?what is that?=SSH dSSH=

Surface Height, Temperature and Chlorophyll: 1 kmSST: 26 Sept. 1998SSH: 2 AltimetersSept. 1998Chl-a Pigment:26-27 Sept. 1998

Global Map of Seafloor Topography Based on Altimetryhttp://ibis.grdl.noaa.gov/cgi-bin/bathy/bathD.pl

Wind Stress (“tau(tau” - τ)Active measurement, using microwave radar to get “vectorwinds” – speed and direction.• Pulse sent from satellite to ocean surface, then scattereddepending on surface roughness• Surface roughness (capillary waves) depends on windstress• Strength of return to satellite gives wind stress anddirection

Components of an active radar “scatterometer” systemFor wind: Transmit at an anglesource and sensorsignalraw datacalibration/validationprocessing /dissemination

Altimeter and Scatterometer

Future Technologies“Next generation” satellite sensors are being developed by NASA and NOAA.• “Swath altimeters” will provide higher resolution SSH fields, closer to thecoast.• “Interferometer” scatterometers will provide higher resolution wind fields,closer to the coast.• “Hyper-spectral” ocean color sensors will sample the full visible radiationspectrum, allowing the identification of different types of phytoplankton,including those responsible for “harmful algal blooms” that close regions tothe harvest of shellfish. These will also have higher resolution to retrievedata farther up into rivers and estuaries.• SST sensors will combine the IR and Microwave channels to provide allweatherSST fields with higher resolution.• These satellite data and in-water data from subsurface “observatories” willbe “assimilated” into 3-D models of the ocean’s circulation to providepredictions of currents, temperatures, oxygen (to warn of “dead zones”).Need more skilled people to analyze data from these systems!!

SummaryOceanography has traditionally faced a sampling limitation.Satellites allow us to observe large areas quickly, but:Only see the ocean surface;Careful data calibration required for long term data sets.Parameters include: SST, Surface Height, Winds, Ice, Chlorophyll,Fluorescence (Productivity).In the Atmosphere: Profiles: Temperature, Water Vapor, Rain, Ozone, CO 2 .Recent Advances: Salinity (this year), Wind and Height data closer to thecoast and higher resolution; All-Weather SST; Hyperspectral dataNew technologies collect subsurface data in the water – gliders, AUV.Smaller and more power efficient sensors, more chemistry, more biology.Computer models use all of these to predict accurate currents andtemperaturesNeed skilled people to analyze data and advance the science!!

Web SitesOSU/COAS: Cooperative Institute for Oceanographic Satellite Studieshttp://cioss.coas.oregonstate.edu/CIOSS/OSU/COAS/Ocean Optics/Color:http://picasso.coas.oregonstate.edu/ORSOO/U. Wisconsin Madison: Coop. Institute for Meteorological Satellite Studieshttp://cimss.ssec.wisc.edu/Jet Propulsion Laboratory (JPL/NASA) Physical Oceanography Data Centerhttp://podaac.jpl.nasa.gov/NOAA Satellite Research & West Coast Satellite Data: “CoastWatch”http://www.star.nesdis.noaa.gov/star/ & http://coastwatch.pfel.noaa.gov/NASA Goddard Science Center – Satellite Data and Info & Ocean Colorhttp://disc.sci.gsfc.nasa.gov/ & http://oceancolor.gsfc.nasa.gov/NASA “Remote Sensing Tutorial:http://rst.gsfc.nasa.gov/Front/overview.html

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A swath altimeter will sample over a wider band, not just directly under the satellite

Coverage by a single “Wide Swath” altimeter.Simulated estimated heights by a single “Wide Swath” altimeter.

Standard Scatterometer Coverage of Winds off S. Cal25 April 2001

Enhanced Scatterometer Coverage of Winds off S. Cal (Experimental)25 April 2001

Next Generation Scatterometer Coverage of Winds off S. Cal?

Other ‘remote’ sensingMoorings, gliders, floats and AUVsCritical problem in oceanography of undersamplingLarge ships are expensive (~$12-20K per day)Ocean is vast – may take weeks to get on-siteExtremely difficult to sample even small areasSatellites help, but only sample the surfaceNeed more automated subsurface sampling to complementthe satellites!! Then we need ocean forecast models.Challenges: Power, cost and hostile environment(salt water, fouling)Could potentially make huge advances in global coverage

Ocean observing technology: Old school

Ocean observing technology: New school – “Floats”

Awesome global coverage!Imagine these floats with bio/chem sensors

New technology: Autonomous underwater vehicles,Powered and Unpowered

Gliders

Future cabledobservatoriesand gliderpaths offOregon-Washingtoncombinemoorings,gliders,satellites andcomputermodels

Computer Models of 3-D Currents and Temperature:Fishermen at Coos Bay use these fields to direct cruises8‐day composites of GOES SST (right) and MODIS chlorophyll (left) over radarsurface velocities during the period leading up to the forecast. (Middle) 2‐dayforecast SST and surface velocity (without data assimilation).

Computer Models of 3-D Currents and TemperatureAlongtrack altimeter SSH data (dots) are used to “correct” themodel. GOES satellite SST is used to verify the improvement of themodel SST fields.prior (free‐run) modelSSTmodel SST after SSHassimilationGOES daily SST (7/20/08),independent of model.57

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Advanced Very High Resolution• Passive measurement• Obscured by clouds• Requires averaging toget good spatialcoverage• Image at right is 8 days,Jul 2006Radiometer (AVHRR)

Microwave SST:Sees through clouds, but obscured by rainUse of passive microwave is increasing as we develop the technology tomake use of these very weak signals.

Measuring chlorophyll from spaceThe Sea-viewing Wide Field of view Sensor: SeaWiFSThe instrumentThe satellite

Ocean observing technology: New school – “Floats”

Instrumentaion along the Newport LineAll Are Cabled (Except Inshore)